Hydrogen (H2) has been produced in an amount of about five hundred billion Nm3 all over the world. The hydrogen has attracted much attention as future clean energy as well as has been applied for a variety of uses such as refinement of oil or production of ammonia. For example, a fuel cell is capable of efficiently supplying electricity when the hydrogen is supplied externally thereto.
However, the hydrogen is highly reactive gas, so that it is difficult to be transported and stored. Therefore, there has been a need for a safe and inexpensive transportation and storage technology in order to stably supply the hydrogen. In the field of the fuel cell, there has been a problem that a poisoning substance is by-produced on a surface of an electrode catalyst by the action of carbon monoxide. Thus, there has been a need to supply high purity hydrogen generally containing 10 ppm or less of carbon monoxide.
As a hydrogen storage method, at present, a method for storing hydrogen as high pressure gas in a gas cylinder is commonly used. However, in this method, there are problems of safety upon transportation of the high pressure gas, and hydrogen brittleness of container materials. A method for storing hydrogen gas in the form of liquid hydrogen under extremely low temperature is also used. However, there have been problems that much energy is consumed in a liquefaction process and that the liquid hydrogen is lost in a percentage of 3% per day to 6% per day due to vaporization.
In order to solve the above described problems with regard to hydrogen transportation and storage technologies, there has been considered a method for storing hydrogen as liquid fuel (e.g., methanol and formic acid) which is obtained by hydrogenating carbon dioxide. For example, formic acid (HCOOH) has recently been attracted the attention as a hydrogen storage material since the formic acid, which is in the liquid form at normal temperature and has a relatively low toxicity, can be decomposed into hydrogen (H2) and carbon dioxide (CO2). However, there has been a problem that thermal decomposition of the formic acid using a conventionally known catalyst requires high temperature of 200° C. or higher, and generates carbon monoxide as a by-product. Therefore, there has been a need to develop a catalyst which allows hydrogen to be selectively and efficiently produced from formic acid under a mild condition, and, if necessary, allows high pressure hydrogen to be supplied.
Recently, many catalysts for dehydrogenation of formic acid containing metal complexes have been reported with regard to a hydrogen production technology almost without by-producing carbon monoxide (PTLs 1 to 3 and NPLs 1 and 2). The catalysts can act under a relatively mild reaction condition, but they have an only low activity, which is problematic. Very recently, one of the catalysts left a record of 120,000 in the turnover frequency per hour at 80° C. However, the catalyst was reacted for only 1 min, and required an amine additive in alcohol, so that it is a long way from practical use (NPL 3). Apart from the above reports, the present inventors have found a catalyst for dehydrogenation of formic acid to be used in water. However, there have remained problems with regard to catalyst activity and catalyst durability which are unsatisfactory for practical use, and the use of an expensive catalyst ligand (PTLs 4 to 6 and NPLs 4 to 7).
Recently, the present inventors has been found an epoch-making catalyst which allows hydrogen to be highly efficiently and highly selectively produced through dehydrogenation of formic acid in water (turnover frequency per hour: 230,000; reaction temperature: 90° C.) under a mild reaction condition without using an organic additive in a cooperative research with Brookhaven National Laboratory in USA (PTL 7). However, there has remained an economic problem since this catalyst contains a catalyst ligand which is synthesized in a complicated manner.
On the other hand, there has been considered that a catalyst for dehydrogenation of formic acid is applied to a technology for producing heavy hydrogen gas which is used for producing an expensive heavy-hydrogenated compound. The heavy-hydrogenated compound has been widely utilized as, for example, a label compound in investigation of a mechanism of a reaction through tracing the reaction or in structural analysis of a biological substance. Recently, it has also attracted the interest as pharmaceuticals, agricultural chemicals, organic EL materials, or optical fibers. However, production of the heavy-hydrogenated compound has conventionally needed to include a number of steps, so that the resultant heavy-hydrogenated compound was very expensive, and only limited types of heavy-hydrogenated compounds were obtained. Extremely expensive deuterium gas (D2) is generally used for synthesizing the heavy-hydrogenated compound. Currently, the heavy-hydrogenated gas is produced by electrolysis of heavy water, but application thereof at a laboratory scale is restricted by its extremely high cost.
On the contrary, it has been known that the heavy hydrogen gas (D2, T2) can be produced from hydrogen gas (H2) and heavy water (D2O, T2O), which are both easily available, through an H/D exchange reaction. However, there has been a problem that a time-consuming pretreatment is needed (NPL 8). There has been considered, as a one of technologies for producing the heavy hydrogen gas, a method for producing heavy hydrogen by utilizing a catalyst for dehydrogenation of heavy formic acid in heavy water. However, this technology has a problem that the reaction rate is extremely slow, so that it has remained mere a theoretical examination.
There has been attempted a method for producing hydrogen isotope gas through dehydrogenation of formic acid from an aqueous solution of formic acid in which any one of water and formic acid (formic acid salt) has been heavy-hydrogenated. However, there has not been reported that satisfactory catalyst performance is achieved (PTLs 8 and 9 and NPLs 9 to 11).
Under the above described circumstances, there has been a need for a method for producing high purity heavy hydrogen gas in an inexpensive and easier manner.
NPL 12 describes a multinuclear catalyst for production of alcohols through reduction of ketones using formic acid. However, the turnover frequency per hour of the catalyst was very low, i.e., up to 20, so that the catalyst is never expected to be used as a catalyst for dehydrogenation of formic acid.